The instant invention is in the field of particulate filters for trapping particulates such as ash and soot from diesel engine exhaust. More specifically, the instant invention relates to diesel exhaust filters comprising a porous metal filter media.
Diesel engines are fuel efficient and durable. However, diesel engine exhaust is a source of soot pollution. Soot pollution is a public health concern. It will be a significant advance in the diesel engine art when an effective, rugged and cost effective means is devised to significantly reduce diesel engine exhaust soot emissions.
Diesel exhaust soot filters are under development; see, for example, chapters 8 and 9 of the text book entitled Catalytic Air Pollution Control, Commercial Technology, 2nd ed., 2002, ISBN 0-471-43624-0, by Heck and Farrauto, herein fully incorporated by reference. As discussed in Heck and Farrauto, most diesel soot filters use a porous ceramic filter element and can optionally incorporate a catalyst to lower the oxidation temperature of the soot, to control other pollutants such as carbon monoxide or unburned hydrocarbon vapor and even to absorb nitrogen oxides. Porous ceramic filter elements have a number of beneficial properties such as resistance to the high temperatures of diesel exhaust when the diesel engine operated at full power and the even higher temperatures that can occur during filter regeneration, i.e., when the trapped soot is ignited to burn off the soot and regenerate the filter (see, for example, FIG. 2 of U.S. Pat. No. 4,515,758).
The primary disadvantage of ceramic filter elements is their tendency to break in service. Diesel exhaust soot filters incorporating porous metal filter elements have been identified as being more promising than diesel exhaust soot filters incorporating ceramic filter elements because porous metal filter elements are less likely to break in service, see U.S. Pat. No. 5,709,722, herein fully incorporated by reference, and especially Column 2, lines 30–67.
The Environmental Security Technology Certification Program of the United States Department of Defense recently (May 2003) reported (Cost and Performance Report CP-9906) a comparison test of a commercially available diesel soot filter comprising a metal filter element (a mat of metal fibers) and a commercially available diesel soot filter comprising a porous ceramic filter element (porous cordierite).
The porous ceramic filter system demonstrated a 90% reduction in particulate emissions. The porous metal filter system (see U.S. Pat. No. 6,572,682, herein fully incorporated by reference) demonstrated a 62% reduction in particulate emissions. The porous ceramic filter system showed evidence of filter element breakage at the end of the testing. The porous metal filter system showed evidence of filter element gasket failure at the end of the testing. The cost of either system was about the same as the cost of the diesel engines used in the testing.
A number of open cell porous metal structures have been reported; see, for example chapters 1 and 2 of the text book by Ashby et al., Metal Foams, A Design Guide, 2000, ISBN 0-7680-0555-8. Ashby et al., page 5, state that such open cell porous metal structures “have potential for high-temperature gas and fluid filtration”. In 1977 Frank E. Towsley was granted a patent on a unique open cell porous metal structure made, for example, by electrodepositing a metal in the interstitial spaces of a compacted bed of polystyrene particles followed by dissolution of the polystyrene, see U.S. Pat. No. 4,053,371, herein fully incorporated by reference. Towsley used such a porous cellular metal, for example, in an improved electrolytic cell; see U.S. Pat. No. 4,121,992, herein fully incorporated by reference, and Towsley suggested a number of other applications such as a filtration membrane, an electrode assembly for batteries, lightweight structural members, impact energy absorbers, and abrasive grinding combinations. However, Towsley did not teach the use of his porous cellular metal material in a diesel soot filter.
A diesel soot filter must not excessively increase the exhaust back pressure of the diesel engine. As discussed by Heck and Farrauto (see, for example, Section 3.2 of Chapter 9, of Heck and Farrauto) a diesel soot filter having an average pore size in the range of from about 10 to about 30 micrometers provides a filtration efficiency of from greater than 90% soot removal to about 60% soot removal for a filter element having a wall thickness of 0.017 inches. If the smaller pore size porous media is selected, then its filtration efficiency is higher but more filter area is needed to maintain a given exhaust back pressure through the filter. Conversely, if the larger pore size porous media is selected, then its filtration efficiency is lower but less filter area is needed to maintain a given exhaust back pressure through the filter. And, if the filter media has a greater porosity (more open pore area at the surface of the filter and more open volume in the filter wall) then back pressure is reduced while filtration efficiency is maintained (assuming the same pore size, the same wall thickness and the same filter area). However, the porosity of porous ceramic filter material is generally not greater than about 35% open pore area at the surface of the filter and about 50% open space in the filter wall (but see the apparently highly porous ceramic foam structure of U.S. Pat. No. 4,965,101) because porous ceramic filter material having a greater porosity tends to be too fragile for use in a diesel soot filter.
It would be an advance in the art of diesel soot filters if a diesel soot filter were developed that incorporated a durable break resistant porous metal filter element but which provided greater soot removal efficiency than existing diesel soot filters employing porous metal filter elements.